The actual book isn’t out until August. An eARC is an Electronic Advanced Reader Copy. This is the super early version that most publishers only make available for reviewers, but Baen are devout capitalists so you too can get the early, unedited, manuscript.
People always ask me how different the eARCS are from the final books. For some authors the difference is significant. For me, not so much. My manuscripts are usually pretty clean and I’ve rarely had to make any big changes between the eARC and the final (other than copy editing/typos).